Photosensitive lithographic printing plate and method for making a printing plate

ABSTRACT

A photosensitive lithographic printing plate includes a photosensitive layer and a protective layer formed in this order on a support, wherein the photosensitive layer has a maximum peak of spectral sensitivity within a wavelength range ranging from 390 to 430 nm, the minimum exposure for the photosensitive lithographic printing plate for image formation at a wavelength of 410 nm (S410) is at most 100 μJ/cm 2 , and the relation between the minimum exposure for image formation at a wavelength of 450 nm (S450) and the minimum exposure for image formation at a wavelength of 410 nm (S410) is 0&lt;S410/S450≦0.1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photosensitive lithographic printingplate which is sensitive to a laser light having a wavelength rangingfrom 390 to 430 nm and which is suitable for direct drawing from digitalsignals of e.g. computers, and a method for making a printing plate.

2. Description of the Related Art

Heretofore, a lithography method has been widely used for themicrofabrication of e.g. lithographic printing plates, printed boards,color filters, large scale integrated circuits (LSI), thin filmtransistors (TFT), liquid crystal displays (LCD), plasma display panels(PDP) and semiconductor packagings (TAB), wherein an image formingmaterial having a layer of a photosensitive composition formed on thesurface of a support is image-exposed through a mask, to form a patternwhich utilizes the difference in solubility of a developer betweenexposed portions and non-exposed portions of the photosensitive layer.

For example, a photopolymerization initiation system for the resistmaterial for color filters, is a combination of a hexaarylbiimidazolederivative with an aminobenzophenone derivative as taught, for example,in JP-A-11-327127.

However, the resist material for color filters disclosed inJP-A-11-327127 is not intended to be exposed to laser light ranging from390 to 430 nm, but rather is subjected through a mask to the light fromthe like of a high-pressure mercury-vapor lamp. Further, since thematerial contains a large amount of a pigment, a large quantity ofenergy of 200 mJ/cm², for example, is required for image formation.

Further, U.S. Pat. No. 5,863,678 discloses a resist material for colorfilters containing a titanocene compound and a dialkylaminobenzenecompound as a photopolymerization initiation system. The above patentdiscloses various kinds of light sources. However, the resist materialis also exposed to light through a mask from a high pressure mercuryvapor lamp source, practically, and the exposure energy of the light ishigh, for example 70 mJ/cm² or more.

Further, the above patent does not disclose safe light properties undera yellow lamp for a photosensitive lithographic printing plate.

On the other hand, the laser direct drawing method has attractedattention in recent years directly forming an image from digitalinformation supplied by a computer, without using a mask, by employing alaser light as a light source for exposure, since improvement in notonly productivity but resolution and accuracy of position and the like.Accordingly, the utilization of a laser light in the lithography methodhas been actively studied.

With respect to the laser light, various light sources emitting lightfrom the ultraviolet to infrared region are known. Potential laser lightfor use in image exposure, is light in the visible to infrared regionemitted by devices, such as an argon ion laser, a helium-neon laser, aYAG laser and a semiconductor laser. These devices are mentioned fromthe viewpoint of output, stability, photoperceptivity, cost and thelike. For example, various photosensitive compositions have beenproposed for use with an argon ion laser having a wavelength of 488 nmand a FD-YAG laser having a wavelength of 532 nm, and a photosensitivelithographic printing plate for exposure to light from these lasers hasbeen placed into practical use. Examples of such photosensitivecompositions are a combination of a titanocene compound with abipyromethene complex and a combination of titanocene with a coumarinderivative as described, for example, in JP-A-11-271969.

However, no conventional compositions are known which are responsiveupon exposure to a violaceous laser light having a wavelength of from390 to 430 nm.

Further, in an image formation method utilizing such visible laserlight, a photosensitive composition showing an adequate absorption atthe visible region is used. However, the sensitivity of the compositionto violaceous laser light having a wavelength ranging from 390 to 430 nmtends to be inadequate, and safe light properties under a yellow lamp(under environment containing a light having a wavelength of from about500 to about 750 nm) tend to be poor in some cases, and accordingly theoperation has to be conducted in a dark room environment using a redlamp.

On the other hand, JP-A-61-117549 discloses, as a photosensitivelithographic printing plate which has excellent in safe lightproperties, a photosensitive lithographic printing plate having aprotective layer containing a specific coloring agent formed on aphotopolymerizable photosensitive composition layer. It discloses as anexposure source, general-purpose light sources such as high-pressuremercury-vapor lamps and metal halide lamps, and argon ion lasers,helium-cadmium lasers and krypton lasers as well, and it discloses as apolymerization initiator a complex system of biimidazole and Michler'sketone. However, JP-A-61-117549, specifically discloses that acomposition having a relative sensitivity to light of a wavelength at480 nm when exposed to a xenon lamp, and a composition responsive toradiation energies required for image formation at 488 nm using an argonion laser and a composition responsive to light of 442 nm the case ofusing a helium-cadmium laser, attracted attention. The publicationdiscloses that no attention has been paid to compositions responsive tolaser light having an oscillation wavelength of from 390 to 430 nm,particularly from 400 to 420 nm. It also discloses that a compositionresponsive to radiation energy required for image formation of 0.6 to1.0 mJ/cm² to light from a helium-cadmium laser having a wavelength of442 nm is not yet sufficient.

On the contrary, along with significant progress in laser technology inrecent years, a laser emitting light in the ultraviolet region, whichcan operate in a bright room environment such as under a yellow lamp,particularly a semiconductor laser which can stably oscillate in awavelength region of from 390 to 430 nm, particularly from 400 to 420nm, has been developed. However, no photosensitive lithographic printingplate suitable for exposure to laser within the violaceous light region(light ranging from 390 to 430 nm in wavelength) has yet been found.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention to provide a highlysensitive photosensitive lithographic printing plate which is responsiveto violaceous laser light.

Another object of the present invention is to provide a method formaking a printing plate using the photosensitive lithographic printingplate.

Briefly, these objects and other objects of the present invention ashereinafter will become more readily apparent can be attained by aphotosensitive lithographic printing plate comprising a photosensitivelayer and a protective layer formed in this order on a support, whereinthe photosensitive layer has a maximum peak of spectral sensitivitywithin a wavelength range ranging from 390 to 430 nm, the minimumexposure for the photosensitive lithographic printing plate for imageformation at a wavelength of 410 nm (S410) is at most 100 μJ/cm², andthe relation between the minimum exposure for image formation at awavelength of 450 nm (S450) and the minimum exposure for image formationat a wavelength of 410 nm (S410) is 0<S410/S450≦0.1.

A second aspect of the present invention is a photosensitivelithographic printing plate comprising a photosensitive layer and aprotective layer formed in this order on a support, the photosensitivelayer containing (A) an ethylenic monomer, (B) a sensitizing pigment and(C) a radical generator, wherein the radical generator (C) contains ahexaarylbiimidazole compound or a titanocene compound, and thesensitizing agent (B) contains a dialkylaminobenzene compound.

A third aspect of the present invention is a method for making aprinting plate, which comprises image-exposing the photosensitivelithographic printing plate according to the first or second aspect ofthe present invention, by means of a laser light having a wavelength offrom 390 to 430 nm, followed by development of the image by an aqueousdeveloper.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the first aspect of the present invention, the photosensitivelithographic printing plate comprises a photosensitive layer and aprotective layer formed in this order on a support, the photosensitivelayer having a maximum peak of spectral sensitivity within a wavelengthrange ranging from 390 to 430 nm, the minimum exposure for imageformation at a wavelength of 410 nm being at most 100 μJ/cm², and theratio of the minimum exposure for image formation at a wavelength of 410nm (S410) to the minimum exposure for image formation at a wavelength of450 nm (S450), i.e. S410/S450 being at most 0.1.

The maximum peak of spectral sensitivity is more preferably within awavelength range ranging from 400 to 420 nm. If the maximum peak ofspectral sensitivity for a photosensitive composition is in a wavelengthregion less than the above range, the sensitivity to a laser lighthaving a wavelength ranging from 390 to 430 nm (hereinafter sometimesreferred to as a laser light in the violaceous region) tends to be poor.On the contrary, if the maximum peak of spectral sensitivity is in awavelength region exceeding the above range, the sensitivity to laserlight in the violaceous region may be high, but the safe lightproperties of the composition under a yellow lamp tend to be poor.

The minimum exposure for image formation of a composition at awavelength of 410 nm is at most 100 μJ/cm², preferably at most 50μJ/cm², more preferably at most 35 μJ/cm². If the minimum exposure ismore than 100 μJ/cm², the exposure time tends to be long, such beingimpractical, although that depends on the exposure intensity of thelaser light source.

Here, the lower the lower limit, the better, but it is usually at least1 μJ/cm², and it is at least 2.5 μJ/cm² practically.

In the present invention, the “maximum peak of spectral sensitivity” ofa composition may, for example, be determined by a method disclosed ine.g. “Photopolymer Technology” (Tsugio Yamaoka, Nikkan Kogyo Shinbunsha,1988, page 262) as follows. Samples of a photosensitive image-formingmaterial having a layer consisting of a photosensitive composition (aphotosensitive layer) formed on the surface of a support, are irradiatedwith light emitted from a light source such as a xenon lamp or atungsten lamp for exposure, using a spectral sensitivity measuringapparatus (Here, the samples are irradiated so that the exposurewavelength linearly decreases in the horizontal axis direction, and theexposure intensity logarithmically decreases in the vertical axisdirection.), followed by developing the samples to obtain a resist imagedepending upon each exposure wavelength, whereupon the exposure energyrequired for image formation is calculated from the height of the image.The exposure wavelength showing the maximum peak in a spectralsensitivity curve obtained by plotting the wavelength on the horizontalaxis and the inverse of the above exposure energy on the vertical axis,corresponds to the maximum peak of spectral sensitivity.

Further, the minimum exposure for image formation at a wavelength of 410nm is determined as an exposure energy calculated from the height of theimage obtained in the same manner as described above using a spectralsensitivity measuring apparatus.

Here, the minimum exposure for image formation is the minimum exposurerequired to form an image when the photosensitive layer is exposed anddeveloped under optimum development conditions determined by changing,e.g., the developer, such as by changing the pH of the developer, thedevelopment temperature or the development time depending upon the typeof photosensitive composition used. The minimum exposure is usually theminimum exposure required to form an image by soaking the photosensitivelayer in an alkali developer having a pH ranging from 11 to 14 at 25° C.for from 30 seconds to 3 minutes after exposure.

Further, with respect to the photosensitive lithographic printing plateof the present invention, the ratio of the minimum exposure for imageformation at a wavelength of 410 nm (S410: J/cm²) to the minimumexposure for image formation at a wavelength of 450 nm (S450: J/cm²)i.e. S410/S450 is at most 0.1. The value of S410/S450 is preferably atmost 0.05. That is, when the minimum exposure S450 for image formationat a wavelength of 450 nm is large and the minimum exposure S410 forimage formation at a wavelength of 410 nm is small, handling efficiencyof the composition tends to be excellent in an environment in which thelight has a wavelength in the vicinity of 500 nm, i.e. under a yellowlamp, and accordingly the smaller the above ratio, the better. Here, theminimum of S410/S450 is 0, which indicates that S450 is infinitelylarge. That is, the photosensitive layer is completely insensitive to alight having a wavelength of 450 nm.

Further, from the viewpoint of handling efficiency under a yellow lamp,the minimum exposure for image formation at a wavelength longer than 450nm is preferably larger than the minimum exposure for image formation ata wavelength of 450 nm. Specifically, the minimum exposure for imageformation at each of wavelength exceeding 450 nm and at most 750 nm ispreferably larger than the minimum exposure for image formation at awavelength of 450 nm (S450). It is particularly preferred that themaximum peak of spectral sensitivity within a wavelength range rangingfrom 390 to 430 nm of the photosensitive layer is the maximum peak ofspectral sensitivity within a wavelength range ranging from 390 to 750nm.

The above photosensitive lithographic printing plate is particularlyuseful when exposed to laser light having an oscillation wavelength offrom 400 to 420 nm.

The constituency of the photosensitive layer in the photosensitivelithographic printing plate is not particularly limited so long as itsatisfies the above definition. However, preferably a photopolymerizablecomposition is used which contains an ethylenic compound and aphotopolymerization initiation system (a combination of a sensitizingagent with a radical generator), which has an advantage over a silversalt type photosensitive composition in view of handling efficiency inwaste disposal after development or the like.

The photosensitive lithographic printing plate of the second aspect ofthe present invention is explained below. The photosensitivelithographic printing plate is a photosensitive lithographic printingplate containing specific constituents to achieve the physicalproperties of the photosensitive layer of the photosensitivelithographic printing plate of the first aspect of the presentinvention.

Ethylenic Monomer

In the present invention, the ethylenic monomer (A) is a compound havinga radically-polymerizable ethylenic double bond which undergoes additionpolymerization by the action of a photopolymerization initiation system,when the photosensitive composition is irradiated with active lightradiation, and which undergoes crosslinking and curing in some cases. Itmay be a compound having one ethylenic double bond in a molecule, andspecifically, it may, for example, be an unsaturated carboxylic acidsuch as acrylic acid, methacrylic acid (hereinafter (meth)acrylic),crotonic acid, isocrotonic acid, maleic acid, itaconic acid orcitraconic acid, an alkyl ester thereof, (meth)acrylonitrile,(meth)acrylamide or styrene. However, preferred is a compound having atleast two ethylenic double bonds in a molecule from the viewpoint ofpolymerizability, crosslinking properties and the resulting increase indifference in solubility in a developer of the exposed portions andnon-exposed portions of a photosensitive composition.

Suitable ethylenic monomers include (A-1) an ester of an unsaturatedcarboxylic acid with an aliphatic polyhydroxyl compound, (A-2) aphosphate containing an acryloyloxy group or a methacryloyloxy group,(A-3) a urethane(meth)acrylate or (A-4) an epoxy (meth)acrylate.

The ester of an unsaturated carboxylic acid with an aliphaticpolyhydroxyl compound (A-1) may be an ester of the above-mentionedunsaturated carboxylic acid with an aliphatic polyhydroxyl compound suchas ethylene glycol, diethylene glycol, triethylene glycol, tetraethyleneglycol, propylene glycol, trimethylene glycol, 1,3-butanediol,tetramethylene glycol, neopentyl glycol, hexamethylene glycol,trimethylolethane, trimethylolpropane, glycerol, pentaerythritol,dipentaerythritol or sorbitol. Specific examples of the ester includeethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate,triethylene glycol di(meth)acrylate, tetraethylene glycoldi(meth)acrylate, propylene glycol di(meth)acrylate, 1,3-butanedioldi(meth)acrylate, tetramethylene glycol di(meth)acrylate, neopentylglycol di(meth)acrylate, hexamethylene glycol di(meth)acrylate,trimethylolethane tri(meth)acrylate, tetramethylolethanetri(meth)acrylate, trimethylolpropane tri(meth)acrylate, glyceroldi(meth)acrylate, glycerol tri(meth)acrylate, pentaerythritoldi(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipentaerythritol di(meth)acrylate,dipentaerythritol tetra(meth)acrylate, dipentaerythritolpenta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, sorbitoltri(meth)acrylate, sorbitol tetra(meth)acrylate, sorbitolpenta(meth)acrylate and sorbitol hexa(meth)acrylate, and similarcrotonates, isocrotonates, maleates, itaconates and citraconates.

The phosphate containing an acryloyloxy group or a methacryloyloxy group(A-2) embodiment is not particularly limited so long as it is aphosphate compound having a (meth)acryloyloxy group in its structure.Particularly preferred phosphates are those represented by the followingformulas (Ia) or and (Ib):

wherein R¹ is a hydrogen atom or a methyl group, n is an integer rangingfrom 1 to 25, and m is 1, 2 or 3.

Here, n preferably ranges from 1 to 10, particularly preferably from 1to 4, and specific examples include methacryloyloxy ethyl phosphate,bis(methacryloyloxy ethyl)phosphate and methacryloyloxy ethylene glycolphosphate. These compounds may be used alone or as a mixture.

The urethane (meth)acrylate (A-3) may, for example, be aurethane(meth)acrylate of a polyisocyanate compound such as an aliphaticpolyisocyanate such as hexamethylene diisocyanate ortrimethylhexamethylene diisocyanate, an alicyclic polyisocyanate such ascyclohexane diisocyanate or isophorone diisocyanate or an aromaticpolyisocyanate such as tolylene diisocyanate, xylylene diisocyanate ordiphenylmethane diisocyanate, with an unsaturated hydroxyl compound suchas hydroxymethyl(meth)acrylate, hydroxyethyl(meth)acrylate, glyceroldi(meth)acrylate, pentaerythritol tri(meth)acrylate ortetramethylolethane tri(meth)acrylate. Specifically, hexamethylenebis[(meth)acryloyloxy methylurethane], hexamethylenebis[(meth)acryloyloxy ethylurethane], hexamethylenebis{tris[(meth)acryloyloxymethyl]methylurethane} or hexamethylenebis{tris[(meth)acryloyloxymethyl]ethylurethane} may, for example, bementioned.

The epoxy (meth)acrylate (A-4) may, for example, be anepoxy(meth)acrylate of a polyepoxy compound such as (poly)ethyleneglycol polyglycidyl ether, (poly)propylene glycol polyglycidyl ether,(poly)tetramethylene glycol polyglycidyl ether, (poly)pentamethyleneglycol polyglycidyl ether, (poly)neopentyl glycol polyglycidyl ether,(poly)hexamethylene glycol polyglycidyl ether, (poly)trimethylolpropanepolyglycidyl ether, (poly)glycerol polyglycidyl ether or (poly)sorbitolpolyglycidyl ether, with a hydroxy(meth)acrylate compound such ashydroxymethyl(meth)acrylate or hydroxyethyl(meth)acrylate.

Further, another ethylenic monomer may, for example, be an ester of anunsaturated carboxylic acid with an aromatic polyhydroxyl compound suchas hydroquinone di(meth)acrylate, resorcin di(meth)acrylate orpyrogallol tri(meth)acrylate, a (meth)acrylhydroxyl compound such as a(meth)acryloyl ethylene oxide addition product of ethylene glycol or a(meth)acryloyl diethylene oxide addition product of2,2-bis(4-hydroxyphenyl)propane, or a condensate of a polyhydroxylcompound, an unsaturated carboxylic acid and a polyhydric carboxylicacid, such as a condensate of ethylene glycol, (meth)acrylic acid andphthalic acid, a condensate of diethylene glycol, (meth)acrylic acid andmaleic acid, a condensate of pentaerythritol, (meth)acrylic acid andterephthalic acid, or a condensate of butanediol, glycerol,(meth)acrylic acid and adipic acid. Further, an amide of theabove-mentioned unsaturated carboxylic acid with an aliphatic polyaminecompound such as methylene diamine, ethylene diamine, diethylenetriamine or hexamethylene diamine, specifically, methylenebis(meth)acrylamide, ethylene bis(meth)acrylamide, diethylene triaminetris(meth)acrylamide or hexamethylene bis(meth)acrylamide may, forexample, be mentioned.

A preferred ethylenic monomers is a (meth)acryloyloxy group-containingphosphate, and the content of the (meth)acryloyloxy group-containingphosphate in the photosensitive composition preferably ranges from 1 to60 wt %, particularly preferably from 2 to 40 wt %, based on the entireethylenic monomer content of the photosensitive layer. Within thisrange, exposure sensitivity and printing resistance of thephotosensitive layer tend to improve, and developing properties of thecomposition tend to improve (stain at a non-image portion willdecrease).

Another preferred ethylenic monomer is urethane (meth)acrylates, andamong the urethane (meth)acrylates, a urethane compound (a3) having atleast four urethane linkages [—NH—(C═O)—O—] and at least fouraddition-polymerizable double bonds in one molecule is preferred. Themethod of producing the urethane compound is not particularly limited,but preferably a compound (a1) having at least four active isocyanategroups in one molecule is reacted with a compound (a2) having at leastone hydroxyl group and at least two addition-polymerizable double bondsin one molecule, since a urethane linkage can easily be formed by theaddition reaction of an isocyanate group (—N═C═O) with a hydroxyl group.

As to the question of why a composition exhibits improved sensitivity bythe addition of the urethane type compound (a3) to a composition, thefollowing may be considered. For example, the formation of activeurethane radicals, because of a chain transfer reaction orphotopolymerization of multi-functional acrylate groups in the urethanetype compound (a3), induces a high photo-setting action, and thisphoto-setting action is increased by the high molecular weight of theurethane compound (a3).

Compound (a1) having at least four active isocyanate groups in onemolecule may, for example, be a compound having at least four activeisocyanate groups in the molecule which are introduced by the reactionof a compound having at least two alcoholic hydroxyl groups (hereinafterreferred to as polyhydric alcohol) with a compound having at least twoisocyanate groups. Specific examples of compound (al) are prepared byreacting a compound having at least four alcoholic hydroxyl groups inone molecule such as pentaerythritol or polyglycerol with a diisocyanatecompound such as hexamethylene diisocyanate, toluene diisocyanate,isophorone diisocyanate or trimethyl hexamethylene diisocyanate; or acompound prepared by reacting a compound containing at least twoalcoholic hydroxyl groups in one molecule such as ethylene glycol with acompound containing at least three isocyanate groups in one moleculesuch as a biuret compound including Duranate 24A-100, 22A-75PX, 21S-75Eand 18H-70B manufactured by Asahi Chemical Industry Co., Ltd. or anadduct compound including P-301-75E, E-402-90T and E-405-80Tmanufactured by Asahi Chemical Industry Co., Ltd.

Further, a compound having at least four isocyanate groups on theaverage in one molecule may be prepared by homopolymerization of anisocyanate of ethylmethacrylate or by copolymerization of the compoundwith another component. Specific examples of the compound (a1) having atleast four isocyanate groups in one molecule include Duranate ME20-100(trade name, manufactured by Asahi Chemical Industry Co., Ltd.)

The number of isocyanate groups in compound (a1) is preferably at least6, particularly preferably at least 7. If the number of isocyanategroups is less than 4, the sensitivity of the resulting compositiontends to be poor. The upper limit of isocyanate groups is notparticularly limited, but if the number is too high, synthesis tends tobe difficult, and accordingly the number of isocyanate units ispreferably at most 20. The number of the isocyanate groups may beadjusted by the number of hydroxyl groups in the polyhydric alcohol, thetype of compound having at least two isocyanate groups and the blendingratio.

The molecular weight of compound (a1) is usually at least 500,preferably at least 1,000, and at most 200,000, preferably at most150,000. If the molecular weight is beyond this range, sensitivity ofthe photosensitive composition tends to decrease.

Compound (a2) having at least one hydroxyl group and at least twoaddition-polymerizable double bonds in one molecule, constitutingurethane compound (a3), may, for example, be a compound having at leastone alcoholic hydroxyl group, prepared by esterification of a compoundhaving a plurality of alcoholic hydroxyl groups such as a polyhydricalcohol with a compound containing a carboxyl group and a (meth)acryloylgroup, i.e. a reaction product prepared by reacting the above carboxylgroup-containing compound in such a proportion that at least onealcoholic hydroxyl group remains. More specifically, compound (a2) maybe a hydroxyl group-containing multi-functional acrylate compound havingat least one alcoholic hydroxyl group, which is an ester of a polyhydricalcohol with acrylic acid, such as a compound prepared by reaction of 3mols of acrylic acid with 1 mol of pentaerythritol, a compound preparedby reaction of 2 mols of acrylic acid with 1 mol of pentaerythritol, acompound prepared by reaction of 5 mols of acrylic acid with 1 mol ofdipentaerythritol, or a compound prepared by reaction of 4 mols ofacrylic acid with 1 mol of dipentaerythritol, and specific examplesinclude pentaerythritol triacrylate, pentaerythritol diacrylate,dipentaerythritol diacrylate, dipentaerythritol triacrylate,dipentaerythritol tetraacrylate and dipentaerythritol pentaacrylate.Such a compound may be used alone or as a mixture in a photosensitivecomposition.

Other examples of compound (a2) include the reaction product of acompound containing an epoxy group such as glycidyl methyl ether with acompound having at least one carboxyl group and at least oneaddition-polymerizable double bond; a reaction product of a compoundhaving at least one epoxy group and at least one addition-polymerizabledouble bond with a compound containing at least one carboxyl group; anda reaction product of a compound having at least one carboxyl group andat least one addition-polymerizable double bond with a compound havingat least one epoxy group and at least one addition-polymerizable doublebond. An example of such a reaction product is the reaction product ofglycidyl methacrylate with (meth)acrylic acid. Preferred compounds (a2)are those which have at least three addition-polymerizable double bonds,in view of the sensitivity of product photosensitive compositions.

The reaction of compound (a1) with compound (a2), i.e. the reaction ofthe isocyanate group in compound (a1) with the hydroxyl group incompound (a2), to prepare the urethane compound (a3), may be conductedin accordance with a known method. Specifically, the isocyanate group incompound (a1) and the hydroxyl group in compound (a2) are reacted in aproportion of 1/10 to 10. For example, both compounds can be dissolvedin an organic solvent such as toluene or ethyl acetate, followed byheating the solution at a temperature ranging from 10 to 150° C. for 5minutes to 30 hours. Another suitable method requires the addition of acatalyst such as n-butyl tin dilaurate in a required amount to asolution of the reactants. Still another method is to first dilutecompound (a2) in a proper organic solvent and then dropwise add compound(a1) thereto. Yet another satisfactory method is the reverse

The molecular weight of urethane compound (a3) is preferably at least600. If it is at most 600, the layer consisting of the photosensitivecomposition (non-cured film) tends to exhibit poor fastness. On theother hand, the upper limit is not particularly limited, but it ispreferably at most 150,000 in view of ease of synthesis andavailability. The molecular weight may be adjusted on the basis of thetypes of compounds (a1) and (a2) and the degree of esterification.

Urethane compound (a3) has at least four addition-polymerizable doublebonds in view of the desired sensitivity of the product photosensitivecomposition, but it has more preferably at least six, particularlypreferably at least eight, double bonds.

The urethane type compound (a3) may be used alone or as a mixture.Particularly when the material of the urethane compound is a mixture,the urethane compound (a3) is used as a mixture of reaction products.

In the production of urethane compound (a3), a functional group otherthan the addition-polymerizable double bond may be introduced into themolecule for the purpose of controlling various properties of thephotosensitive composition. For example, a compound (a4) having ahydroxyl group and a carboxyl group in one molecule in combination withcompound (a2) may be reacted with compound (a1) to prepare a compoundhaving at least four urethane linkages, at least fouraddition-polymerizable double bonds and a carboxyl group in onemolecule.

More specifically, a compound (a1′) having at least four isocyanategroups in one molecule, a compound (a2′) having one hydroxyl group andtwo addition-polymerizable double bonds in one molecule and a compound(a4′) having one hydroxyl group and one carboxyl group in one moleculemay be reacted at a molar ratio of a1′:a2′:a4′ of 1:3:1 to produce acompound (a3′) having four urethane linkages, and six double bonds andone carboxyl group on the average, in one molecule.

As the compound (a4) for reaction with compound (a1) in combination withcompound (a2) (the compound having a hydroxyl group and a carboxyl groupin one molecule), specifically, an aliphatic carboxylic acid having ahydroxyl group, such as 2-hydroxyoctanic acid, 2-hydroxyhexanoic acid,2-hydroxydecanoic acid, 3-hydroxyoctanoic acid or 8-hydroxyoctanoicacid, is preferred, and a C₄₋₂₀ carboxylic acid having a hydroxyl groupat the a-position of the carboxylic acid is particularly preferred. Asthe urethane compound (a3), preferred is a urethane compound representedby the following formula (II):

wherein x is an integer ranging from 4 to 20, y is an integer rangingfrom 0 to 15, z is an integer ranging from 1 to 15, Ra is a group havinga repeating unit derived from alkyleneoxy or aryleneoxy, and having from4 to 20 oxy groups which are capable of combining with Rb, each of Rband Rc which are independent of each other, is a C₁₋₁₀ alkylene group,and Rd is an organic residue having from one to ten (meth)acrylic group,provided that each of Ra, Rb, Rc and Rd which are independent of oneanother, may have a substituent.

The alkyleneoxy group which is present in the repeating unit in Ra, maybe an alkyleneoxy residue of, e.g. glycerol, pentaerythritol orpropylene triol, and the aryleneoxy group which is present in therepeating unit in Ra, may be a phenoxy residue of, e.g. pyrogallol or1,3,5-benzene triol.

It is preferred that x be an integer ranging from 4 to 15, y be aninteger ranging from 1 to 10, z be an integer ranging from 1 to 10, eachof Ra and Rc which are independent of each other, is a C₁₋₅ alkylenegroup, and Rd is an organic residue having from one to seven(meth)acrylic groups.

More preferably, Ra is

wherein k is from 2 to 10, each of Rb and Rc, which are independent ofeach other, is —C₂H₄—, —CH₂—C(CH₃)— or —C₃H₆—, and Rd is

The compounding ratio of urethane compound (a3) ranges from 0.5 to 50parts by weight, preferably from 1 to 40 parts by weight, morepreferably from 2 to 30 parts by weight, based on 100 parts by weight ofthe ethylenic monomer.Photopolymerization Initiation System

The photopolymerization initiation system usually contains a radicalgenerator (C) and a sensitizing agent (B) and further contains ahydrogen-donor compound as a polymerization accelerator as the caserequires. The radical generator (C) is a compound which receives lightexcitation energy from the sensitizing agent (B) when the sensitizingagent is irradiated with active light radiation, generates activeradicals by the absorbed energy, and causes the above ethylenic monomerto undergo polymerization. The components of the photosensitive layer ofthe photosensitive lithographic printing plate of the second aspect ofthe present invention comprise a hexaarylbiimidazole compound or atitanocene compound as the radical generator (C), and adialkylaminobenzene compound as the sensitizing agent (B).

The hexaarylbiimidazole compound may be a dimer of an imidazole compoundhaving three aryl groups which generates radicals directly by exposureto a laser ranging in wavelength from 390 to 430 nm or by interactionwith the copresent sensitizing agent. The hexaarylbiimidazole compoundmay, for example, be a hexaarylbiimidazole compound as disclosed in,e.g. JP-B-45-37377, JP-A-47-2528 or JP-A-54-15529, and specifically, itmay, for example, be 2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole,2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(p-methoxyphenyl)biimidazole,2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(p-methylphenyl)biimidazole,2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(p-ethoxycarbonylphenyl)biimidazole,2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(p-fluorophenyl)biimidazole,2,2′-bis(o-bromophenyl)-4,4′,5,5′-tetra(p-iodophenyl)biimidazole,2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(p-chloronaphthyl)biimidazole,2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(p-chlorophenyl)biimidazole,2,2′-bis(o-bromophenyl)-4,4′,5,5′-tetra(p-chlor-p-methoxyphenyl)biimidazole,2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(o,p-dichlorophenyl)biimidazole,2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(o,p-dibromophenyl)biimidazole,2,2′-bis(o-bromophenyl)-4,4′,5,5′-tetra(o,p-dichlorophenyl)biimidazole,or2,2′-bis(o,p-dichlorophenyl)-4,4′,5,5′-tetra(o,p-dichlorophenyl)biimidazole.Of these compounds, preferred is a hexaphenylbiimidazole compound. Aparticularly preferred hexaphenylbiimidazole compound is one which hasthe ortho-positions of the benzene rings at the 2,2′-positions on theimidazole rings substituted by halogen, and still more preferred is onewhich in addition has the benzene rings at the 4,4′,5,5′-positions onthe imidazole rings not substituted, substituted by halogen orsubstituted by alkoxycarbonyl.

Such a hexaarylbiimidazole may be used in combination with variousbiimidazoles as the case requires. The biimidazoles can easily besynthesized by the method disclosed in Bull. Chem. Soc. Japan. 33,565(1960) or J. Org. Chem. 36[16]2262 (1971).

Suitable titanocene compounds include titanium compounds having adicyclopentadienyl structure and a biphenyl structure, such asdicyclopentadienyl titanium dichloride, dicyclopentadienyl titaniumbisphenyl, dicyclopentadienyltitanium bis(2,4-difluorophenyl),dicyclopentadienyl titanium bis(2,6-difluorophenyl), dicyclopentadienyltitanium bis(2,4,6-trifluorophenyl), dicyclopentadienyl titaniumbis(2,3,5,6-tetrafluorophenyl), dicyclopentadienyl titaniumbis(2,3,4,5,6-pentafluorophenyl), di(methylcyclopentadienyl) titaniumbis(2,6-difluorophenyl), di(methylcyclopentadienyl) titaniumbis(2,3,4,5,6-pentafluorophenyl) or dicyclopentadienyl titaniumbis[2,6-difluoro-3-(1-pyrrolyl)phenyl], may, for example, be mentioned,and a preferred compound is one having the o-positions of the biphenylring replaced with a halogen atom.

Further, it is possible to use, as a radical generator, the abovehexaarylbiimidazole compound and the titanocene compound in combination.

The dialkylaminobenzene type compound which is used as the sensitizingagent (B) may have any optional substituent so long as it is a compoundwhich has a dialkylaminobenzene structure, absorbs light having awavelength ranging from 390 to 430 nm, and efficiently generatesradicals from the radical generator by interaction with the radicalgenerator. Preferred is a dialkylaminobenzophenone compound, adialkylaminobenzene compound having an aromatic heterocyclic group as asubstituent on the carbon atom at the p-position relative to the aminogroup on the benzene ring, or a compound having a nitrogen-containingheterocyclic structure formed by linkage of the alkyl groupsconstituting the dialkylamino group in said compound and/or by linkageof said alkyl group with the carbon atom on the benzene ring adjacent tothe carbon atom to which the amino group is bonded. Here, the aminogroups constituting the dialkylamino group may be the same or different,and have a carbon number preferably ranging from 1 to 6.

Of these compounds, particularly preferred are dialkylaminobenzenecompounds represented by the following formulae (IIIa) and (IIIb):

wherein each of R² to R⁵, which are independent of one another, is aC₁₋₆ alkyl group, and each of R⁶ to R⁹ is a hydrogen atom or a C₁₋₆alkyl group, provided that R² and R³, R⁴ and R⁵, R² and R⁶, R³ and R⁷,R⁴ and R⁸, or R⁵ and R⁹, may be bonded to each other to form a ring;

wherein each of R¹⁰ and R²¹, which are independent of each other, is aC₁₋₆ alkyl group, each of R¹³ and R¹⁴ which are independent of eachother, is a hydrogen atom or a C₁₋₆ alkyl group, Y is a sulfur atom, anoxygen atom, dialkylmethylene or —N(R¹⁵)—, and R¹⁵ is a hydrogen atom ora C₁₋₆ alkyl group, provided that R¹⁰ and R¹¹, R¹⁰ and R¹³, or R¹¹ andR¹⁴, may be bonded to each other to form a ring. Here, the carbon numberof each alkyl group in the dialkylmethylene ranges from 1 to 6,preferably 1.

In a case where a pair of two of R² to R¹⁴ is bonded to form a ring,preferred is a 5- or 6-membered ring, and particularly preferred is a6-membered ring.

Suitable compounds represented by formula (IIIa), include4,4′-dimethylaminobenzophenone, 4,4′-diethylaminobenzophenone and thefollowing compounds:

The compound having formula (IIIb) may be2-(p-dimethylaminophenyl)benzooxazole,2-(p-diethylaminophenyl)benzooxazole,2-(p-dimethylaminophenyl)benzo[4,5]benzooxazole,2-(p-dimethylaminophenyl)benzo[6,7]benzooxazole,2,5-bis(p-diethylaminophenyl)1,3,4-oxazole,2-(p-dimethylaminophenyl)benzothiazole,2-(p-diethylaminophenyl)benzothiazole,2-(p-dimethylaminophenyl)benzimidazole,2-(p-diethylaminophenyl)benzimidazole or the following compound:

As a dialkylaminobenzene compound other than those of the formulae(IIIa) and (IIIb), 2,5-bis(p-diethylaminophenyl)1,3,4-thiadiazole,(p-dimethylaminophenyl)pyridine, (p-diethylaminophenyl)pyridine,2-(p-dimethylaminophenyl)quinoline, 2-(p-diethylaminophenyl)quinoline,2-(p-dimethylaminophenyl)pyrimidine or2-(p-diethylaminophenyl)pyrimidine may, for example, be mentioned.

Here, in view of the important factor of the handling efficiency of aphotosensitive composition under a yellow lamp, particularly in orderthat the ratio of S410/S450 is at most 0.1, preferred is aphotopolymerization initiation system consisting of a combination of thehexaarylbiimidazole compound as the radical generator (C) with thedialkylaminobenzene compound as the sensitizing agent (B).

Further, the photosensitive composition of the present invention, whichis to be exposed to violaceous light, preferably contains ahydrogen-donor compound component as a polymerization accelerator inaddition to the above components, for the purpose of improvingphotopolymerization initiation performance.

Specific examples of the hydrogen-donor compound include compoundshaving a mercapto group such as 2-mercaptobenzothiazole,2-mercaptobenzimidazole, 2-mercaptobenzoxazole and3-mercapto-1,2,4-triazole, N,N-dialkyl benzoic alkyl ester,N-aryl-a-amino acids, their salts and esters such as N-phenylglycine,salts of N-phenylglycine, and alkyl esters of N-phenylglycine such asN-phenylglycine ethyl ester and N-phenylglycine benzyl ester, andcompounds represented by the following formula (IV):

wherein R¹⁶ is a hydrogen atom or an alkyl group which may have asubstituent, R¹⁷ is a hydrogen atom, an alkyl group which may have asubstituent, a vinyl group which may have a substituent, an aryl groupwhich may have a substituent, a (meth)acryloyl group which may have asubstituent, an aryl group which may have a substituent or an aromaticheterocyclic group which may have a substituent, and the benzene ring ofthe compound may have a substituent, and p is a integer ranging from 2to 10.

The photosensitive lithographic printing plate of the present inventionpreferably contains a polymer binder in addition to the above componentsin the photosensitive layer, for the purpose of improving, e.g.development properties or coating properties, when the photosensitivelayer is coated on a substrate.

Specific examples of the polymer binder include homopolymers andcopolymers of, e.g. (meth)acrylic acid, (meth)acrylic ester,(meth)acrylamide, maleic acid, (meth)acrylonitrile, styrene, vinylacetate, vinylidene chloride and maleimide, and polyethylene oxide,polyvinyl pyrrolidone, polyamide, polyurethane, polyester, polyether,polyethylene terephthalate, acetyl cellulose and polyvinyl butyral.

Of these binder materials, preferred is a copolymer containing carboxylgroups in its molecule and containing, as copolymerizable components,(meth)acrylic acid and at least one (meth)acrylate which may besubstituted, such as methyl(meth)acrylate, ethyl(meth)acrylate,propyl(meth)acrylate, butyl(meth)acrylate, 2-ethylhexyl(meth)acrylate,hydroxyethyl(meth)acrylate or benzyl(meth)acrylate (hereinafter referredto as “carboxyl group-containing copolymer”).

The acid value of the carboxyl group-containing polymer binderpreferably ranges from 10 to 250 KOH·mg/g, and the weight averagemolecular weight calculated as polystyrene (hereinafter referred tosimply as Mw) preferably ranges from 5,000 to 1,000,000, more preferablyfrom 10,000 to 500,000. Such a polymer binder preferably has unsaturatedbonds in its side chains, and a resin obtained by reacting a compoundhaving both epoxy group and unsaturated group with the above carboxylgroup-containing copolymer may be mentioned.

Suitable compounds having both an epoxy group and an unsaturated groupinclude aliphatic epoxy group-containing unsaturated compounds such asallyl glycidyl ether, glycidyl(meth)acrylate,a-ethylglycidyl(meth)acrylate, glycidyl crotonate, glycidylisocrotonate, crotonyl glycidyl ether, monoalkyl monoglycidyl itaconate,monoalkyl monoglycidyl fumarate or monoalkyl monoglycidyl maleate, or3,4-epoxycyclohexylmethyl(meth)acrylate. Of these compounds, preferredis allyl glycidyl ether, glycidyl(meth)acrylate or3,4-epoxycyclohexylmethyl(meth)acrylate, and more preferred is3,4-epoxycyclohexylmethyl(meth)acrylate.

Also preferred is a compound having the structural unit (monomer unit)represented by the following formula (V):

wherein Re is a hydrogen atom or a methyl group.

Of the above-mentioned resins having both carboxyl groups and doublebonds in their side chains, a preferred molecular weight, Mw ranges from10,000 to 1,000,000, preferably from 20,000 to 500,000. Further,preferred is a resin having from 1 to 50, preferably from 5 to 40 unitsof double bonds introduced into the side chains based on 100 monomerunits of the main chain.

The photosensitive lithographic printing plate of the present inventionpreferably contains, in the photosensitive layer, an amine compoundhaving a pKb (dissociation constant) of at most 7 at 25° C. or an aminocompound having an atomic group [N—CH₂] in its molecule, for the purposeof improving sensitivity to laser light having a wavelength within arange ranging from 390 to 430 nm. The photosensitive lithographicprinting plate more preferably contains, in the photosensitive layer, anamine compound having a pKb of at most 7 at 25° C. and has the group[N—CH₂] in its molecule.

The amine compound may be any one of aliphatic, alicyclic and aromaticamines so long as the above conditions are satisfied, and thehydrocarbon group in said amine may have a substituent. Further, theamine compound is not limited to a monoamine, since it may be apolyamine such as a diamine or a triamine, and it may be a primaryamine, a secondary amine or a tertiary amine. However, in view of pKbvalues, an aliphatic amine having a hydrocarbon group which may have asubstituent is, much preferred and a tertiary amine is particularlypreferred.

The pKb value of the amine is preferably at most 5. Further, the lowerlimit of the pKb value is preferably at least 3. Further, an aminehaving the group [CH₂—N—CH₂] in its molecule is more preferred.

Specifically, an aliphatic amine which may be replaced with a hydroxylgroup or a phenyl group, such as butylamine, dibutylamine,tributylamine, amylamine, diamylamine, triamylamine, hexylamine,dihexylamine, trihexylamine, benzylamine, dibenzylamine, tribenzylamine,triethanolamine, allylamine, diallylamine or triallylamine, may bementioned.

As the amine compound for use in the present invention, since itpractically has to remain in the photosensitive layer when thephotosensitive layer is coated and dried, and since it has to be usedwithout presenting any problems in handling such as odor, a preferredamine compound is one which has a boiling point of at least 80° C. undernormal pressure, and particularly preferred is one which has a boilingpoint of at least 150° C. and which is solid at room temperature (25°C.). Further, a triaralkyl amine is preferred since it does not decreasethe dispersibility of the coloring pigment. From this viewpoint, andtaking availability into consideration, tribenzylamine is a particularlypreferred example.

The reasons for improvement in sensitivity as a result of addition ofthe amine are believed to be as follows.

(1) The formation of active amino radicals by a chain transfer reactionof an amine with radicals generated in the photopolymerizationinitiation mechanism or polyacrylate radicals formed byphotopolymerization of an acrylate monomer by the action of theradicals.

(2) In a sensitizing step by electron transfer from a light excitationsensitizing agent to a radical generator, the sensitizing agent cationand radical generator anion are formed, but in general, a reverseelectron transfer to the pigment cation deactivates the radicalgenerator anion. The amine transfers electron to the sensitizing agentcation and changes the cation to a neutral sensitizing agent, and itthereby suppresses the reverse electron transfer, increasesdecomposition efficiency of the radical generator anion, and increasesthe radical generation effect.

(3) The amine significantly increases the elution rate particularly ofthe non-image portion or a photosensitive layer in an inadequately curedstate to an alkali developer at the time of alkali development, and itthereby prevents loss of the photo-set photosensitive layer andincreases sensitivity.

For the photosensitive layer of the photosensitive lithographic printingplate of the second aspect of the present invention, a combination ofthe above ethylenic monomer, the hexaarylbiimidazole compound (radicalgenerator) and the dialkylaminobenzene compound (sensitizing agent), asthe photopolymerization initiation system, or a combination of the aboveethylenic monomer, the titanocene compound (radical generator) and thedialkylaminobenzene compound (sensitizing agent) is essential. Thecompounding ratio of each component of the composition, based on 100parts by weight of the ethylenic monomer, is such that the amount of thehexaarylbiimidazole compound preferably ranges from 5 to 60 parts byweight, more preferably from 15 to 40 parts by weight, the amount of thetitanocene compound preferably ranges from 1 to 30 parts by weight, morepreferably from 5 to 20 parts by weight, and the amount of thedialkylaminobenzene compound preferably ranges from 1 to 30 parts byweight, more preferably from 5 to 20 parts by weight. Further, thecompositional ratio of the sensitizing agent to the radical generator issuch that the dialkylaminobenzene compound preferably ranges from 0.1 to5 parts by weight, more preferably from 0.2 to 3 parts by weight, basedon 1 part by weight of the hexaarylbiimidazole compound, and thedialkylaminobenzene compound ranges from 0.5 to 6 parts by weight, morepreferably from 0.5 to 1.8 parts by weight, based on 1 part by weight ofthe titanocene compound.

Further, in the case where the polymerization accelerator(hydrogen-donor compound) is incorporated in the composition for thepurpose of improving photopolymerization initiation performance, it isincorporated in an amount preferably ranging from 1 to 50 parts byweight, more preferably from 10 to 40 parts by weight, based on 100parts by weight of the ethylenic monomer component.

Further, in the case where the polymer binder is incorporated in thecomposition, it is incorporated in an amount preferably ranging from 50to 500 parts by weight, more preferably from 70 to 200 parts by weight,based on 100 parts by weight of the ethylenic monomer.

The photosensitive layer of the lithographic printing plate to be usedin the present invention may contain another substance depending uponthe purpose for its use. For example, a coating property-improving agentsuch as a nonionic, anionic, cationic or fluorine type surface activeagent; a thermal polymerization inhibitor such as hydroquinone,p-methoxyphenol or 2,6-di-t-butyl-p-cresol; a coloring pigment of anorganic or inorganic dye or pigment (which is different from the abovesensitizing agent, which is substantially incompatible with a coatingsolvent or a photosensitive layer component, and which has nosensitizing function); a plasticizer such as dioctyl phthalate,didodecylphthalate or tricresyl phosphate; a sensitivityproperties-improving agent such as a tertiary amine or a thiol; oranother additive such as a pigment precursor, an antifoaming agent, avisible image-imparting agent, an adhesion-improving agent, adevelopment property-improving agent or an ultraviolet absorber, may beadded.

With respect to preferred amounts of the above additives based on 100parts by weight of the ethylenic monomer, the thermal polymerizationinhibitor is at most 2 parts by weight, the coloring pigment is at most20 parts by weight, the plasticizer is at most 40 parts by weight, thepigment precursor is at most 30 parts by weight, and the surface activeagent is at most 10 parts by weight.

Particularly, if the content of the coloring pigment is too high, theperformance of the present invention may be inadequate. Accordingly, thecontent of the coloring pigment is preferably at most 20 wt % of thephotosensitive composition.

The above photosensitive composition is diluted with a proper solvent,and coated and dried on a support to form the photosensitive layer.

The support which is used in the present invention may be any of theconventional ones used for photosensitive lithographic printing plates,and it may, for example, be a metal plate of, e.g., aluminum, zinc,iron, copper or an alloy thereof, a metal plate having chromium, zinc,copper, nickel, aluminum, iron or the like plated or vapor-depositedthereon, a paper sheet, a plastic film, a glass sheet, a resin-coatedpaper sheet, a paper sheet having a metal foil such as an aluminum foilbonded thereto, or a plastic film having hydrophilic treatment appliedthereto. Among them, preferred is a plate of aluminum or an aluminumalloy (hereinafter referred to as an aluminum support).

The thickness of the aluminum support is usually ranges from about 0.01to about 10 mm, preferably from about 0.05 to about 1 mm.

The surface on at least the photosensitive layer composition side of thealuminum support is subjected to surface roughening, and then an anodicoxidation treatment is conducted. A degreasing treatment, a sealingtreatment, an undercoating treatment or the like may further beconducted as the case requires. A degreasing treatment is conductedusually before the surface roughening, and the degreasing treatment isconducted in accordance with a conventional method such as wiping,soaking or steam-washing the support with a solvent, by soaking orspraying the support with an aqueous alkali solution, followed byneutralization with an aqueous acid solution, or by soaking or sprayingthe support with a surface active agent. Surface roughening may beaccomplished by a known such as brush polishing, ball polishing,electrolytic etching, chemical etching, liquid honing or sand blasting,or a combination thereof. Preferred is brush polishing, ball polishing,electrolytic etching, chemical etching or liquid honing.

The aluminum plate which is surface is further subjected to a desmuttingtreatment with an aqueous acid or alkali solution as the case requires.The desmutting treatment is conducted by soaking the plate in an aqueoussolution of an acid such as sulfuric acid, nitric acid, hydrochloricacid, phosphoric acid or chromic acid, or in an aqueous solution of analkali such as sodium hydroxide, potassium hydroxide, sodiummetasilicate, sodium phosphate, sodium pyrophosphate, potassiumphosphate or sodium aluminate, or by spraying aqueous solutions onto theplate. The aluminum plate thus obtained is usually subjected to ananodic oxidation treatment, particularly preferably a treatment with anelectrolytic solution containing sulfuric acid. The method of anodicoxidation treatment with an electrolytic solution containing sulfuricacid may be conducted in accordance a known method such as disclosed inJP-A-58-213894. Specifically, it is conducted, for example, at asulfuric acid content ranging from 5 to 50 wt %, preferably from 15 to30 wt %, at a temperature ranging from about 5 to about 50° C.,preferably from 15 to 35° C., at a current density ranging from 1 to 60A/dm² for from about 5 to about 60 seconds. Further, as the caserequires, the substrate surface may be treated with hot water or with analkali silicate such as a silicate of soda, or by soaking the plate inan aqueous solution containing an aqueous polymer compound such aspolyvinyl phosphonic acid or a resin having a cationic quaternaryammonium group. The thickness of the aluminum support usually from 0.01to 10 mm, preferably from about 0.05 to about 1 mm, and with respect tothe surface roughness, the average roughness Ra as stipulated in JISB0601 usually ranges from 0.3 to 1.0 μm, preferably from about 0.4 toabout 0.8 μm.

The photosensitive lithographic printing plate used in the presentinvention is exposed by means of a laser light relatively low exposure.Accordingly, it is effective to leave smut on the surface of thesubstrate so as to increase the adhesion of the photosensitivecomposition, within a range not influence such as staining duringprinting or impairing development properties. The residual amount of thesmut preferably ranges from 0.3 to 0.5, more preferably from 0.32 to0.45, as by the reflection density on the surface the photosensitivecomposition side. When the reflection ratio is within the above range,printing resistance will improve.

Further, in the case where the desmutting treatment is conducted, it ispreferred to control the desmutting conditions so that 0.01≦D-E≦0.1,where D is the reflection density on the surface at the photosensitivecomposition side immediately after the surface roughening treatment, andE is the reflection density on the surface the photosensitivecomposition side after the anodic oxidation treatment. More preferably,0.01≦D-E≦0.08.

The reflection density is measured by means of a reflection densitometervisual mode without using a filter. The desmutting of a surface may beperformed by soaking the substrate in an aqueous NaOH solution at aconcentration preferably ranging from 0.1 to 4 wt %, at a liquidtemperature ranging from about 5 to about 30° C. for from about 1 toabout 10 seconds, depending on the state of the surface roughening andthe aqueous alkali solution used.

The photosensitive composition may be coated on a surface by a knownmethod such as by dip coating, coating by means of a rod, spinnercoating, spray coating or roll coating. The amount coated ranges from0.5 to 5 g/m². Here, the temperature for drying ranges from about 30 toabout 150° C., preferably from about 40 to about 110° C., and the dryingtime ranges from about 5 seconds to about 60 minutes, preferably fromabout 10 seconds to about 30 minutes.

A protective layer (oxygen-shielding layer) is provided on thephotosensitive layer in order to prevent polymerization inhibition dueto the presence of oxygen. Specific examples of protective layers arethose formed from water-soluble polymers such as a polyvinyl alcohol,polyvinyl pyrrolidone, polyethylene oxide and cellulose. Among them,preferred is a polyvinyl alcohol having high oxygen gas barrierproperties. It is also preferred to use a polyvinyl alcohol andpolyvinyl pyrrolidone in combination, and in this case, preferably from1 to 20 parts by weight, more preferably from 3 to 15 parts by weight,of polyvinyl pyrrolidone is used based on 100 parts by weight of thepolyvinyl alcohol.

The photosensitive lithographic printing plate of the present inventionmay be formed in a manner such that the photosensitive layer of aphotosensitive composition having a maximum peak of spectral sensitivityoutside the wavelength range from 370 to 430 nm, then a protective layertransmission of light is formed on the photosensitive layer so as toform, for example, a layer made of a photosensitive composition having amaximum peak of spectral sensitivity within a wavelength range rangingfrom 430 to 460 nm, and on photosensitive layer, a protective layershowing absorption within a range ranging from 430 to 500 nm is formed,so that the maximum peak of spectral sensitivity is within a wavelengthrange ranging from 400 to 420 nm, and the absolute sensitivity is atmost 100 μJ/cm² at a wavelength of 410 nm resultingly.

A preferred aspect of the invention is that the photosensitivecomposition which constitutes the photosensitive layer in the abovecase, is one in which the polymerization initiation system is changed,specifically to a combination of the above titanocene compound with acoumarin compound. coumarin pigments as disclosed in e.g. JP-A-6-301208,JP-A-8-146605, JP-A-8-211605, JP-A-8-129258 or JP-A-8-129259. Thecoumarin pigment is a pigment which has the following skeleton in itsstructure.

In the case of forming a photosensitive layer using a photosensitivecomposition which undergoes photo-radical polymerization, anoxygen-shielding layer is preferably formed on the photosensitive layeras mentioned above so as to prevent radical polymerization inhibitiondue to the action of oxygen, and it is possible to employ theoxygen-shielding layer as a protective layer and to adjust thecomponents in the protective layer to adjust the maximum peak ofspectral sensitivity as photosensitive lithographic printing plate towithin a wavelength range ranging from 400 to 440 nm. The protectivelayer may contain a water-soluble polymer similar to one in the aboveoxygen-shielding layer as the main component, and contains a compoundwhich absorbs light having the above desired wavelength.

The third aspect of the invention of making a printing plate using thephotosensitive lithographic printing plate of the present invention isas follows.

In order to prepare the printing plate of the present invention, thelithographic printing plate of the invention is exposed by means of alaser having a wavelength ranging from 390 to 430 nm, followed bydevelopment to remove non-exposed portions of the photosensitive layerto form an image.

The source of light for image formation is not particularly limited longas it is laser light having an oscillation wavelength ranging from 390to 430 nm. Preferred is laser light having an oscillation wavelengthranging from 400 to 420 nm, and particularly advantageous is an indiumgallium nitride semiconductor laser emitting light having a wavelengthin the vicinity of 410 nm.

Exposure of the photosensitive lithographic printing plate is conductedby employing a laser beam having an oscillation wavelength ranging from390 to 430 nm, preferably from 400 to 420 nm, as a beam spot having adiameter ranging from 2 to 30 μm, preferably from 4 to 20 μm, at anoutput light intensity of the laser ranging from 1 to 100 mW, preferablyfrom 3 to 70 mW, and by moving the beam spot at a scanning rate rangingfrom 50 to 500 m/s, preferably from 100 to 400 m/s.

Image-exposure is conducted so that exposure of the photosensitivelithographic printing plate (printing plate exposure) to laser light isat most 100 μJ/cm², preferably at most 50 μJ/cm². The lower limit ispreferably as low as possible, but is usually at least 1 μJ/cm², and ispractically at least 5 μJ/cm². Further, the higher the scanning densityupon exposure, the greater the high-definition image formation.Accordingly, the scanning density is preferably at least 2,000 dpi, morepreferably at least 4,000 dpi.

The photosensitive lithographic printing plate of the present inventionis image-exposed by means of the above light source, followed bydevelopment of the image with an aqueous developer consisting mainly ofwater, preferably with an aqueous solution containing a surface activeagent and an alkali component.

The aqueous solution may further contain an organic solvent or abuffering agent. A preferred buffering agent is an alkali component suchas an inorganic alkali such as sodium silicate, potassium silicate,sodium hydroxide, potassium hydroxide, lithium hydroxide, tribasicsodium phosphate, dibasic sodium phosphate, sodium carbonate, potassiumcarbonate and sodium bicarbonate, an organic amine compound such astrimethylamine, diethylamine, isopropylamine, n-butylamine,monoethanolamine, diethanolamine and triethanolamine. These compoundsmay be used alone or in combination. The pH of the alkali developerusually ranges from about 9 to about 14, preferably from 11 to 14.

Surface active agents include a nonionic surface active agent such as apolyoxyethylene alkyl ether, a polyoxyethylene alkyl aryl ether, apolyoxyethylene alkyl ester, a sorbitan alkyl ester or a monoglyceridealkyl ester, an anionic surface active agent such as an alkyl benzenesulfonate, an alkyl naphthalene sulfonate, an alkyl sulfate, an alkylsulfonate or a sulfosuccinate, or an ampholytic surface active agentsuch as an alkyl betaine or an amino acid. Further, suitable organicsolvents include isopropyl alcohol, benzyl alcohol, ethyl cellosolve,butyl cellosolve, phenyl cellosolve, propylene glycol and diacetonealcohol.

The method of development employed is not particularly limited, and maybe conducted by soaking and shaking the plate in a developer, physicallyremoving non-image portions which is at the point of being dissolved ina developer by means of e.g. a brush, or spraying a developer onto theplate so as to remove non-image portions. The time for development isselected depending upon the above method used so that the non-imageportions can adequately be removed, and is optionally selected within arange of 5 seconds to 10 minutes.

After the development, the plate may be subjected to a hydrophilictreatment by means of, e.g., gum arabic optionally applied to theprinting plate as the case requires. Further, the oxygen-shielding layermay be initially washed with water before development as the caserequires.

Having now generally described this invention, a further understandingcan be obtained by reference to certain specific examples which areprovided herein for purposes of illustration only and are not intendedto be limiting unless otherwise specified.

Preparation of an Aluminum Support (1)

An aluminum plate having a thickness of 0.3 mm was degreased with a 3 wt% aqueous sodium hydroxide solution, subjected to electrolytic etchingin a hydrochloric acid bath of 11.5 g/l at 25° C. at a current densityof 80 A/dm² for 11 seconds, and washed with water. Then, the plate wassubjected to anodic oxidation in a sulfuric acid bath of 30 wt % at 30°C. at 11.5 A/dm2 for 15 seconds, washed with water and dried to preparean aluminum plate as a lithographic printing plate (hereinafter referredto simply as support (1)).

Preparation of an Aluminum Support (2)

An aluminum plate (thickness: 0.24 mm) was degreased with a 3 wt %aqueous sodium hydroxide solution, and subjected to electrolytic etchingin a nitric acid bath of 18.0 g/l at 25° C. at a current density of 90A/dm² for 11 seconds. Then, the plate was subjected to desmutting in a4.5 wt % aqueous sodium hydroxide solution at 30° C. for 2 seconds,neutralized in a 10 wt % aqueous nitric acid solution at 25° C. for 5seconds, washed with water, and then subjected to an anodic oxidationtreatment in a nitric acid bath of 30 wt % at 30° C. at a currentdensity of 10 A/dm² for 16 seconds, and washed with water and dried toprepare a support.

The reflection density of the support (measured by a reflectiondensitometer (RD-918 manufactured by Macbeth) was 0.32. Further, duringthis production step, A-B=0.08 (where A is the reflection density on thesurface at the photosensitive composition side immediately after thesurface roughening treatment, and B is the reflection density on thesurface at the photosensitive composition side immediately before theanodic oxidation treatment).

Hereinafter the support thus obtained will be referred to simply as“support (2)”.

EXAMPLES 1 TO 15 AND COMPARATIVE EXAMPLES 1 TO 5

A coating fluid of the following photosensitive composition was coatedonto support (1) by means of a bar coater and dried so that the driedfilm thickness became 2 g/m² (drying condition: 170° C., 2 minutes).Further, the following protective layer coating fluid (1) was coatedthereon by means of a bar coater and dried so that the dried filmthickness became 3 g/m² (drying condition: 170 C, 2 minutes) to preparea photosensitive lithographic printing plate. Protective layer coatingfluid (1) Polyvinyl alcohol (GL-03 manufactured by 90 parts by weightNippon Synthetic Chemical Industry Co., Ltd.) Polyvinyl pyrrolidone (Mw= 4,000) 5 parts by weight Water 1,000 parts by weight

Photosensitive composition coating fluid Radical generator (compound asAmount as shown in Table 1 identified in Table 1) Sensitizing agent(compound as Amount as shown in Table 1 identified in Table 1) Polymerbinder (the following 45 parts by weight compound P-1) Ethylenic monomer1 (the following Amount as shown in Table 1 compound E-1) Ethylenicmonomer 2 (the following 22 parts by weight compound E-2) Ethylenicmonomer 3 (the following Amount as shown in Table 1 compound E-3)Ethylenic monomer 4 (the following Amount as shown in Table 1 compoundE-4) 2-mercaptobenzothiazole 5 parts by weight N-phenylglycinebenzylester Amount as shown in Table 1 Tribenzylamine Amount as shown inTable 1 Copper phthalocyanine pigment 4 parts by weight (visible imageagent) Emulgen 104 P (surface active agent, 2 parts by weightmanufactured by Kao Corporation) S-381 (fluorine type surface active 0.3part by weight agent, manufactured by Asahi Glass Company, Limited)Disperbyk 161 (dispersing agent, 2 parts by weight manufactured by BigChemie) Propylene glycol monomethyl ether 400 parts by weight acetateCyclohexanone 740 parts by weight

Among the components of the photosensitive composition, the structuresof the radical generator, the sensitizing agent, the polymer binder andthe ethylenic monomer are as follows:Radical Generator

Sensitizing Agent

Polymer Binder

Ethylenic Monomer

The obtained photosensitive lithographic printing plate for violaceouslaser exposure was evaluated with respect to the following items. Theresults are shown in Table 1.

Evaluation of Sensitivity

The prepared photosensitive lithographic printing plate was cut into asize of 50×60 mm, and the photosensitive material sample was irradiatedfor 10 seconds with a light of which light intensity logarithmicallydecreased in the vertical axis, and the light wavelength linearlydecreased in the horizontal axis, as determined by means of adiffraction spectral irradiation apparatus (RM-23, manufactured byNarumi K.K.) equipped with xenon lamp U1-501C (1 kW: manufactured byUshio Inc.) as a light source. The exposed sample was soaked in anaqueous solution containing 0.7 wt % of sodium carbonate and 0.5 wt % ofan anionic surface active agent (Pelex NBL, manufactured by KaoCorporation) at 28° C. for 30 seconds to develop the image. Afterdevelopment, the minimum amount of exposure energy required forphotocuring by light radiation of 410 nm was calculated from the heightof the cured image obtained. The smaller amount of energy, the higherthe sensitivity.

S410/S450

The photosensitive material sample was exposed and developed in the samemanner as described in the above evaluation of sensitivity, whereuponthe minimum exposure energy S410 (μJ/cm² ) at a wavelength of 410 nm andthe minimum exposure energy S450 (μJ/cm²) at a wavelength of 450 nm wereobtained to calculate the ratio (S410/S450).

Here, the symbols A to D in the column of minimum exposure energy inTable 1 represent the following.

A: S410/S450 is at most 0.03

B: S410/S450 exceeds 0.03 and is at most 0.1

C: S410/S450 exceeds 0.1 and is at most 0.5

D: S410/S450 exceeds 0.1

The smaller the ratio, the better the safe light properties under ayellow lamp.

Maximum Peak of Spectral Sensitivity

The photosensitive material sample was exposed and developed in the samemanner as described in the above evaluation of sensitivity, whereupon aspectral sensitivity curve was obtained with the horizontal axisindicating the exposure wavelength and the vertical axis indicating theinverse of the minimum exposure energy (sensitivity) at the wavelengthindicated by the horizontal axis, and from the spectral sensitivitycurve, a wavelength showing the maximum peak of spectral sensitivity wasobtained. Here, the exposure wavelength was changed from 350 nm to 650nm.

Safe Light Properties Under a Yellow Lamp

The photosensitive lithographic printing plate was cut into a size of30×30 mm, and each sample was left to stand under a yellow lamp (underthe condition of being shielded from light having wavelengths of at mostabout 470 nm) for 1 minute, 2 minutes, 5 minutes, 10 minutes, 20minutes, 30 minutes or 40 minutes, followed by development in the samemanner as described above, to obtain the maximum time for which thephotosensitive composition left to stand under a yellow lamp would notcure (evaluation of 40 minutes at the most).

Here, symbols A to D in the column of safe light properties in Table 1mean the following.

A: At least 20 minutes

B: At least 10 minutes and less than 20 minutes

C: At least 1 minute and less than 10 minutes

D: Less than 1 minute

EXAMPLE 15

A photosensitive lithographic printing plate was prepared in the samemanner as described in Example 12 by successively coating and drying thephotosensitive composition coating fluid and the protective layercoating fluid, except that the support (2) was used instead of thesupport (1), and evaluations were conducted in the same manner. Theresults are shown in Table 1. TABLE 1 Amount of Amount of Amount ofRadical Sensitizing Absorp- ethylenic ethylenic ethylenic generatorpigment tion monomer-1 monomer-3 monomer-4 (part by (part by maximum(part by (part by (part by weight) weight) (nm) weight) weight) weight)Ex. 1 R-1 (15) S-1 (9) 410 (22) (11)  (0) Ex. 2 R-1 (10) S-1 (6) 410(22) (11)  (0) Ex. 3 R-3 (5) S-1 (3) 410 (22) (11)  (0) Ex. 4 R-1 (10)S-1 (6) 410 (22) (11)  (0) Ex. 5 R-1 (10) S-1 (6) 410  (0) (11) (33) Ex.6 R-2 (15) S-1 (9) 410  (0) (11) (22) Ex. 7 R-2 (15) S-1 (6) 410  (0)(11) (22) Ex. 8 R-2 (10) S-1 (6) 410  (0) (11) (22) Ex. 9 R-2 (5) S-1(6) 410  (0) (11) (22) Ex. 10 R-1 (5) S-1 (4.5) 410  (0) (11) (22) Ex.11 R-3 (5) S-1 (3) 410  (0) (11) (22) Ex. 12 R-1 (15) S-1 (9) 410 (11) (6) (22) Ex. 13 R-1 (15) S-1 (9) 410 (11)  (6) (22) Ex. 14 R-3 (15) S-1(9) 410 (11)  (6) (22) Comp. Ex. 1 R-3 (5) S-2 (3) 450  (0) (11) (22)Comp. Ex. 2 R-2 (15) S-4 (9) Not  (0) (11) (22) measured Comp. Ex. 3 R-2(15) S-4 (9) Not (22) (11)  (0) measured Comp. Ex. 4 R-3 (5) S-2 (3) 450(22) (11)  (0) Comp. Ex. 5 R-3 (5) S-2 (3) 450  (0) (11) (33) Ex. 15 R-1(15) S-1 (9) 410 (11)  (6) (22) Amount of N- phenylglycine Amount ofbenzyl ester tribenzylamine S410 Safe light (part by weight) (part byweight) (μJ/cm²) S410/S450 properties Ex. 1 (5) (3) 14 0.026 20 min. Ex.2 (5) (3) 15 B 40 min. Ex. 3 (5) (3) 18 C 40 min. Ex. 4 (5) (0) 22 B 40min. Ex. 5 (5) (3) 24 B 40 min. Ex. 6 (5) (0) 24 0.026 30 min. Ex. 7 (5)(0) 24 0.038 30 min. Ex. 8 (5) (0) 30 0.032 40 min. Ex. 9 (5) (0) 43 Atmost 40 min. 0.021*² Ex. 10 (5) (0) 54 At most 40 min. 0.027*² Ex. 11(5) (0) 54 0.38  1 min. Ex. 12 (3) (5) 15 A A Ex. 13 (8) (3) 22 A A Ex.14 (8) (0) 27 A A Comp. Ex. 1 (5) (0) 24 2.25 Nil Comp. Ex. 2 (5) (0)F⁺³ — — Comp. Ex. 3 (5) (3) F⁺³ — — Comp. Ex. 4 (5) (3) 14 D D Comp. Ex.5 (5) (3) 21 D D Ex. 15 (3) (5) 15 A D*1: “Nil” means that the plate was cured after having been left to standfor 1 minute.*²It was impossible to form an image with an energy of about 2,000μJ/cm² by exposure at a wavelength of 450 nm under the above conditions.⁺³“F” means that it was impossible to form an image with an energy ofabout 2,000 μJ/cm² by exposure at a wavelength of 410 nm under the aboveconditions.

From the evaluation above of spectral sensitivity, the minimum exposurefor image formation for each of the photosensitive material samples ofExamples 1 to 15 at a wavelength longer than 450 nm was higher than thevalue at a wavelength of 450 nm. On the other hand, the minimum exposurefor image formation for the photosensitive material sample ofComparative Example 1 at a wavelength longer than 450 nm was higher thanthe value at a wavelength of 450 nm. Further, each of the photosensitivelayers of Examples 1 to 15 showed a maximum peak at 410 nm within arange ranging from 350 to 650 nm.

EXAMPLE 16

A photosensitive lithographic printing plate prepared in the same manneras described in Example 1 was image-exposed by means of a 410 nmviolaceous laser printing plate exposure apparatus (Cobalt 8)manufactured by Escher Glad at a laser light output of 0.5 mW with alaser beam spot diameter of 12 μm at a scanning density of 5,080 dpi ata scanning rate of 167 m/s. The image-exposed photosensitivelithographic printing plate was developed in the same manner asdescribed in Example 1, whereupon a print having a high quality imagewas obtained. The printing plate exposure energy was 30 μJ/cm².

With respect to a photosensitive lithographic printing plate prepared inthe same manner as described in Examples 1 to 15, an image can be formedby image exposure in the same manner as described in Example 16 by meansof a 410 nm violaceous laser printing plate exposure apparatus (Cobalt8) manufactured by Escher Glad at a laser light output of 0.5 mW with alaser beam spot diameter of 12 μm, at a scanning density of 5,080 dpi ata scanning rate of 167 m/s, followed by development.

The photosensitive lithographic printing plate of the present inventionis highly sensitive upon exposure to laser light ranging from 390 to 430nm, and accordingly an image can efficiently be formed by means of laserlight ranging in wavelength from 390 to 430 nm.

Further, as a preferred embodiment, the composition exhibits excellentsafe light properties under a yellow lamp and exhibits excellenthandling efficiency.

The disclosures of Japanese Patent Application Nos. 2000-117803 filed onApr. 19, 2000; 2000-131995 filed on May 1, 2000; 2000-131996 filed onMay 1, 2000; 2000-364310 filed on Nov. 30, 2000; 2000-368412 filed onDec. 4, 2000; 2000-369415 filed on Dec. 5, 2000 and 2001-016537 filed onJan. 25, 2001, including specification, claims, drawings and summary areincorporated herein by reference.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A photosensitive lithographic printing plate precursor comprising: asupport; and a photosensitive layer; wherein said photosensitive layercontains (A) an ethylenic monomer, (B) a sensitizing agent, and (C) aradical generator; and said sensitizing agent is a dialkylaminobenzenecompound including an aromatic heterocyclic group as a substituent onthe carbon atom at the p-position relative to the amino group on thebenzene ring.
 2. The photosensitive lithographic printing plateprecursor of claim 1, wherein said radical generator (C) comprises ahexaarylbiimidazole compound or a titanocene compound.
 3. Thephotosensitive lithographic printing plate precursor of claim 2, whereinsaid hexaarylbiimidazole compound is present in an amount ranging from15 to 40 parts by weight based on 100 parts by weight of said ethylenicmonomer.
 4. The photosensitive lithographic printing plate precursor ofclaim 2, wherein said titanocene compound is present in an amountranging from 5 to 20 parts by weight based on 100 parts by weight ofsaid ethylenic monomer.
 5. The photosensitive lithographic printingplate precursor of claim 1, wherein said photosensitive layer comprisesa coloring pigment in an amount from 0 to 20 wt. % based on the weightof said photosensitive layer.
 6. The photosensitive lithographicprinting plate precursor of claim 1, wherein said photosensitive layerfurther comprises a polymer binder.
 7. The photosensitive lithographicprinting plate according to claim 6, wherein the polymer binder containsa structural unit represented by the following formula (V):

wherein Re is a hydrogen atom or a methyl group.
 8. The photosensitivelithographic printing plate precursor of claim 1, wherein saidphotosensitive layer further comprises a hydrogen-donor compound.
 9. Thephotosensitive lithographic printing plate precursor of claim 8, whereinsaid hydrogen-donor compound comprises a mercapto group.
 10. Thephotosensitive lithographic printing plate precursor of claim 1, whereinsaid photosensitive layer further comprises an amine compound having apKb of at most 7 at 25° C.
 11. The photosensitive lithographic printingplate precursor of claim 1, wherein said photosensitive layer furthercomprises an amine compound having a group of the formula [N—CH2]. 12.The photosensitive lithographic printing plate precursor of claim 1,wherein said photosensitive layer comprises, as the ethylenic monomer, aurethane compound (a3) having at least four urethane linkages and atleast four addition-polymerizable double bonds in one molecule.
 13. Thephotosensitive lithographic printing plate precursor of claim 12,wherein said urethane compound (a3) has a molecular weight ranging from600 to 200,000.
 14. The photosensitive lithographic printing plateprecursor of claim 12, wherein said urethane compound (a3) isrepresented by formula (II):

wherein x is an integer ranging from 4 to 20, y is an integer rangingfrom 0 to 15, z is an integer ranging from 1 to 15, Ra is a group havinga repeating unit derived from alkyleneoxy or aryleneoxy, and having from4 to 20 oxy groups capable of combining with Rb, each of Rb and Rc,which are independent of each other, is a C₁₋₁₀ alkylene group, and Rdis an organic residue having from 1 to 10 (meth)acrylic group, providedthat each of Ra, Rb, Rc and Rd which are independent of one another, mayhave a substituent.
 15. The photosensitive lithographic printing plateprecursor of claim 12, wherein said urethane compound (a3) as theethylenic monomer (A), is prepared by reacting a compound having atleast four active isocyanate groups in one molecule (a1) with a compoundhaving at least one hydroxyl group and at least twoaddition-polymerizable double bonds in one molecule (a2).
 16. Thephotosensitive lithographic printing plate precursor of claim 15,wherein the compound (a1) having at least four active isocyanate groupsin one molecule has a molecular weight of at least
 500. 17. Thephotosensitive lithographic printing plate precursor of claim 1, whereinsaid photosensitive layer comprises, as the ethylenic monomer (A), aphosphate compound having an acryloyloxy group or a methacryloyloxygroup.
 18. The photosensitive lithographic printing plate precursor ofclaim 1, wherein said photosensitive layer further comprises a surfaceactive agent.
 19. The photosensitive lithographic printing plateprecursor of claim 1, further comprising a protective layer includingpolyvinyl alcohol.
 20. The photosensitive lithographic printing plateprecursor of claim 1, further comprising a protective layer includingpolyvinyl pyrrolidone.
 21. The photosensitive lithographic printingplate precursor of claim 1, wherein said support includes aluminum or analuminum alloy having a roughened surface, and said support has areflection density of at least 0.3.
 22. The photosensitive lithographicprinting plate precursor of claim 1, wherein said support includesaluminum or an aluminum alloy having a roughened surface and an anodizedlayer.
 23. A method for making a printing plate, which comprises:image-wise exposing the photosensitive lithographic printing plateprecursor as defined in claim 1 with laser light having a wavelengthranging from about 390 nm to about 430 nm; and developing the exposedplate with an aqueous developer.
 24. The method for making a printingplate according to claim 23, wherein an exposure energy of the printingplate precursor in the image-wise exposing step at a wavelength of 410nm is at most 100 μJ/cm².
 25. The method for making a printing plateaccording to claim 23, wherein an exposure energy of the printing plateprecursor in the image-wise exposing step at a wavelength of 410 nm isat most 50 μJ/cm².
 25. The method for making a printing plate accordingto claim 23, wherein a relation between a minimum exposure energy forimage formation of the printing plate precursor at a wavelength of 450nm (S450) and a minimum exposure energy for image formation of theprecursor at a wavelength of 410 nm (S410) is 0<S410/S450≦0.1.